Isotropic pitch-based materials for thermal insulation

ABSTRACT

Insulation materials suited to high temperature applications, such as the insulation of furnaces, are formed from a mixture of pitch carbon fibers, such as isotropic pitch carbon fibers, and a binder comprising a solution of sugar in water. The sugar solution is preferably at a concentration of from 20–60% sucrose to yield a low density material having high flexural strength and low thermal conductivity when carbonized to a temperature of about 1800° C.

This is a divisional application of application Ser. No. 10/184,850entitled “Isotropic Pitch-Eased Materials for Thermal Insulation” filedJun. 28, 2002, now U.S. Pat. No. 6,800,364.

BACKGROUND OF THE INVENION Field of the Invention

Discussion of the Art

Thermal insulation materials formed from carbon fibers exhibit excellentresistance to heat flow, even at high temperatures. Commerciallyavailable materials are generally produced from a carbon fiber filler,derived from a cotton, rayon or pitch precursor, and a binder, such as aphenolic resin solution, furfuryl alcohol, or insoluble starch. In onemethod, the binder and fibers are formed into an artifact under vacuumand then heated to high temperatures to carbonize the binder. Forexample, thermal insulation materials have been prepared by combining0.35% of carbonized rayon fibers, 0.35% by weight of an insolublestarch, and 99.3% by weight of water, molding under vacuum, andcarbonizing at 1000° C. The density of the carbonized insulationmaterial ranged from 0.11 to 0.26 g/cm³, compressive strength rangedfrom 1–10.5 kg/cm², and thermal conductivity ranged from 0.066 to 0.11W/m-°K at 538° C. and from 0.577 to 0.793 W/m-°K at 2200° C., measuredin an argon atmosphere.

In another method, hot pressing is used to form the artifact, followedby carbonization. Thermal insulation materials formed by hot pressingtend to have a higher density than vacuum molded materials, and thusthermal conductivities tend to be higher. For example, a hot pressedcomposite formed by combining 50% by weight carbonized rayon fibers and50% by weight phenolic resin binder or starch slurry, hot pressing, andcarbonizing to 1350° C. had a density of 0.31–0.91 g/cm³.

The rigid mat thus formed is then machined into desired shapes and,optionally, sealed or coated, for example, with a phenolic resin.

For insulation of large furnaces, it is desirable for insulationmaterials to be readily removable for replacement. Materials formed fromconventional fibers, such as rayon fiber, do not generally have asufficient structural strength to be formed into boards which can bereplaced periodically. Additionally, for furnaces which operate at hightemperatures, such as induction furnaces used for graphitization, whichoperate at temperatures of up to about 3200° C., an insulation materialhaving a particularly low thermal conductivity and high thermalstability is desired.

Conventional binders, such as phenolic resin solutions and furfurylalcohol, tend to pose environmental problems and evolve potentiallyharmful byproducts during conversion of the resin to carbon duringprocessing. It is also difficult to control the amount of binder-derivedcarbon that is incorporated into the composite.

The present invention provides a new and improved method and insulationmaterial which overcome the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method offorming a thermal insulation material is provided. The method includescombining carbon-containing fibers, which include pitch-based carbonfibers, with a binder which includes a soluble sugar to form a mixture.The mixture is formed into a solid preform having a general shape of thethermal insulation material. The preform is heated to a sufficienttemperature to carbonize the preform and form the thermal insulationmaterial.

In accordance with another aspect of the present invention, aninsulation material is formed by the method described.

In accordance with another aspect of the present invention, a lowdensity thermal insulation article is provided. The article is formedfrom a mixture of isotropic pitch fibers and a sugar binder which hasbeen heated to a sufficient temperature to carbonize the mixture. Thearticle has a density of from about 0.1 to about 0.4 g/cm³.

In accordance with another aspect of the present invention, a method ofproviding thermal insulation for a high temperature radiant heat sourceis provided. The method includes forming an insulation member having athermal conductivity of less than about 0.4 W/m-°K. The forming stepincludes filtering a mixture which includes isotropic pitch carbonfibers and a sugar solution and heating the filtered mixture to atemperature of at least 900° C. to form the member. The member ispositioned adjacent the high temperature radiant heat source to insulatethe heat source.

An advantage of at least one embodiment of the present invention is thatit provides an insulation material having high flexural strength and lowthermal conductivity.

Another advantage of at least one embodiment of the present invention isthat the binder is environmentally safe, posing fewer disposal problemsthan conventional organic binder systems.

Another advantage of at least one embodiment of the present invention isthat it enables the density and other properties of the insulationmaterial to be adjusted by varying the concentration of soluble sugar inthe binder.

Still further advantages of the present invention will be readilyapparent to those skilled in the art, upon a reading of the followingdisclosure and a review of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is side view of a vacuum filtration system according to thepresent invention;

FIG. 2 is side view of an alternative embodiment of a vacuum filtrationsystem according to the present invention;

FIG. 3 is a side view of a centrifugal casting system according to thepresent invention; and

FIG. 4 illustrates an exemplary furnace insulated with the insulationmaterial according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A process for forming a self-supporting, thermal insulation materialsuited for forming insulation board and other structural insulationproducts, includes mixing a reinforcement material, such as carbonizedfibers, with a liquid binder, such as a sugar solution. Excess bindercan be removed, for example, by filtering the mixture of fibers andliquid binder through a filter material, such as a bleeder cloth.

The reinforcement material includes carbon fibers, alone or incombination with other carbonized or carbonizable materials. The fiberspreferably include isotropic pitch-based carbon fibers, either alone ormixed with other carbon fibers. Preferably, at least 80% of the carbonfibers are isotropic pitch carbon fibers, more preferably, at least 95%,and most preferably, 100% by weight of the carbon fibers are derivedfrom isotropic pitch. Isotropic pitch carbon fibers have been found toexhibit a desirable combination of low thermal conductivity and highflexural strength, as compared to other carbon fibers, such aspolyacrylonitrile (PAN)-based carbon fibers and mesophase pitch carbonfibers. For example, the thermal conductivity of carbon fibers derivedfrom isotropic pitch is about 10 W/m-°K in air at 25° C., as compared to100–1000 W/m-°K for carbon fibers derived from mesophase pitch, 8.5–15W/m-°K for PAN-based carbon fibers, and about 10–15 W/m-K forrayon-based carbon fibers. Insulation materials formed from isotropicpitch carbon fibers according to the present method, as large sheets orboards or similar rigid insulation products, have been found to exhibitsufficient strength and insulation properties to make them suited to useas replaceable insulation for high temperature furnaces, and the like.

Isotropic pitch fibers are formed from a pitch having a high carboncontent, preferably over 90%. The pitch is generally formed from coal orpetroleum, although synthetically formed pitches are also contemplated.The pitch is heated to a liquid state (200–300° C.) and spun to formsemi-viscous solid “fibers.” The fibers are stabilized by a processknown as infusibilization, which prevents the fibers from remelting whensubsequently heat-treated. This process includes reacting the fiberswith air at a relatively low temperature. This destroys the order of thefiber structure and inhibits the formation of ordered graphite when thefibers are further heat treated. The fibers are then heated to a finaltemperature of about 800–1200° C. to convert the fibers to carbon.

While isotropic pitch fibers are preferred, it is also contemplated thatall or a portion of the carbon fibers be mesophase pitch carbon fibersor other carbonized fibers, such as those derived from rayon or PAN.Mesophase pitch fibers are formed at a higher temperature than isotropicpitch fibers. Typically, they are derived from a mesophase pitch productby heat treatment or solvent extraction of an isotropic pitch. Thefibers are formed at a temperature of about 300–400° C. and developlong-range order as a result of the ordered mesophase pitch. They thenundergo an infusibilization process to stabilize the ordered mesophasestructure.

The isotropic pitch fibers or other fibers used are preferablycomminuted, for example, by chopping or milling, to an average length ofabout 100 to about 1600 microns, more preferably, from about 400 toabout 800 microns. Optionally, mixtures of fiber lengths are employed.Fibers longer than about 1200 microns have a tendency to ball up andthus are less desirable.

As used herein, the term “fibers” is intended to encompass all elongatecarbon-containing reinforcement materials having a length which is atleast twenty times, more preferably, at least 100 times the fiberdiameter (often referred to as the aspect ratio). The fibers may becomminuted by a process such as chopping and milling. The carbon fiberspreferably have an aspect ratio equal to or greater than 20:1, morepreferably, greater than 100:1, a length of from about 2–30 mm, and adiameter of about 5–15 microns. Carbon fibers may also take the form ofcontinuous filament yarn, chopped yarn, or tape made from continuousfilaments and which are referred to as unidirectional arrays of fibers.Yarns may be woven in desired shapes by braiding or by multidirectionalweaving. The yarn, cloth and/or tape may be wrapped or wound around amandrel to form a variety of shapes and reinforcement orientations.

A particularly preferred carbonized fiber is derived from isotropicpitch and obtained, for example, from under the tradename Carboflex™from AnShan Chemical Co., China. These fibers have a density of about1.6 g/cm³, a diameter of about 12 microns, and are primarily carbon(i.e., greater than 99% carbon). Isotropic pitch fibers are alsoobtainable from Kureha Chemical Industry Co., Ltd., e.g., Kureha M-104T.

The fibers are combined with a liquid binder which holds the fiberstogether during the subsequent processing stages. Preferred binderscomprise an aqueous solution of a soluble sugar, such as amonosaccharide or disaccharide. Exemplary sugars include sucrose,fructose, dextrose, maltose, mannose glucose, galactose, UDP-galactose,and xylose, their soluble polysaccaride equivalents, and combinationsthereof. Sucrose is particularly preferred because of its high cokingvalue. A particularly preferred binder includes about 15 to about 80%sugar, such as sucrose, dissolved in water, more preferably 20–60%sucrose, most preferably about 45–60% sucrose in water. For achieving anoptimal combination of low thermal conductivity, high strength and lowdensity, the 45–60% sucrose concentration was found to be particularlyeffective. As the sugar content increases, the viscosity increases. Athigh sugar concentrations e.g., above about 60% sucrose, improved flowmay be achieved by heating the fiber and binder mixture, for example, toa temperature of about 60° C. or higher.

Sugars have several properties which make them well suited as bindersfor the present application. They are soluble in water over a wideconcentration range and thus the final binder content of the materialcan be precisely controlled. Further, unlike phenolic resins, the sugardoes not tend to begin curing during the filtering process. Thus, anyexcess binder can be recycled for subsequent reuse. Additionally, theconcentration of sugar is readily adjusted by adding more water orsugar, as needed. Disposal of sugar binders does not pose environmentalproblems as does disposal of phenolic binders. Carbon yields of sugarsare relatively low, generally only 25–35% for sugars, as compared toabout 50% for phenolic binders. However, during carbonization, the mainvolatile material released from sugar is water, while phenolic bindersevolve toxic compounds, such as phenol and formaldehyde, during heattreatment. By using sugar as a binder, the costly enviromental controlsused in processing phenolic binders can be avoided.

Optionally, coking additives or other additives may be included in thesugar binder, such as aluminum phosphate or zinc chloride. These act tomaximize the carbon yield.

The binder solution and fibers are mixed together in a ratio of about10–40 parts by weight of binder solution to about 60–40 parts of fiber.In terms of sugar (i.e., not including the water) a preferred ratio isfrom 20–80% by weight sugar: 80–20% by weight fibers, most preferably,about 40% by weight sugar: 60% by weight fibers. For sucrose, which hasa carbon yield of about 35%, this ratio results in a final producthaving about 14% of carbonized sugar and 86% fibers by weight.Preferably, the carbonized sugar content of the final product is betweenabout 10% and about 20% by weight. If the carbonized sugar is too low,the integrity of the final product may be compromised. As theconcentration of carbonized sugar increases, the density tends toincrease, increasing the thermal conductivity of the material andrendering it less well suited for thermal insulation applications.

Excess binder is preferably removed from the mixture prior to furtherprocessing. For example, a mixture of fibers and binder is poured into aform or mold 10 fitted with a filter, such as a cloth 12 (FIG. 1). Theexcess binder is removed by gravity or a vacuum source 14. For example,a pump or water faucet pulls a vacuum on the filter cloth to remove theexcess binder. The fibers build up on the filter cloth 12 and when thedesired thickness is achieved, the fibers and remaining binder areremoved as a preform 16 in the shape of a mat. This method isparticularly preferred for preparing large sheets or boards ofinsulation material.

In another method, a perforated drum 20 is rotated in a bath 22 of thefiber and binder mixture (FIG. 2). A vacuum source 24 applies a vacuumto an interior of the drum and a cylindrical mat 16 of fibers slowlybuilds up on the outside of the drum. Heat is preferably applied duringthe extraction process to aid in removal of excess water from thecomposite. This method is suited to forming cylindrical castings.

In yet another method, the fiber/binder mixture is centrifuged (FIG. 3).For example, a mixture of fibers and binder is fed into the interior ofa drum 30, which is rotated by a motor 32 (FIG. 3). The drum 30 includesa cylindrical foraminous screen 34, clamped between upper and lowerscreen supports 36, 38. A perforated feedstock tube 40 delivers themixture to the interior of the drum 30, where it builds up on a filtercloth 42.

When a layer of the desired thickness of fibers is achieved, the drum 30is disassembled and the cylindrical preform 16 of fibers and remainingbinder is removed. Three to five minutes of extraction (drum rotation)time is typically sufficient to form the preform. This method isparticularly suited to the formation of cylindrical insulation materialshaving high uniformity in thermal conductivity.

For higher density products, light pressure may be applied to thepreform, either during filtration or during a subsequent heating step,although excessive pressure can compromise the insulative properties ofthe finished product. Preferably, the pressure, if applied, does notresult in a final density of the insulation product of more than about0.5 g/cm³.

The preform 16 formed in a filtration process, such as one of the threedescribed above, is heated to a temperature of about 200° C. to 300° C.to drive off water from the binder solution. For example, the preform isheated to about 250° C. with a heating rate of about 10–20° C./hour. Inthe case of the filtration process embodied in FIG. 2, the heating stepmay be carried out while the mat 16 is still on the drum 20.Alternatively, the mat is removed from the filtration system and driedin an oven. The heat converts the sugar in the binder to an infusible,insoluble form. Specifically, heating a carbohydrate leads to chemicalremoval of OH groups in the form of H₂O and formation of a stable carbonand oxygen-containing cured polymer.

It is also contemplated that the filtering step may be eliminated andthat the mixture simply be heated, first to drive off excess water andlater in the heating process, to convert the remaining sugar to apolymeric form.

The preform is then carbonized to a final temperature of about 900° C.to 2000° C. in an inert (non-oxidizing) atmosphere, such as argon toremove all (or substantially all) oxygen and hydrogen and produce acarbonized preform in the shape of a board or cylindrical casting,depending on forming process used. The carbonization temperature isselected according to the end use of the casting and is generally abovethe highest temperature to which the casting is to be subjected in use.This reduces the chance for outgassing during use. For example, thepreform is carbonized to about 1800° C. by heating in an inertatmosphere at a heating rate of about 100° C./hour.

The resulting carbonized preform comprises primarily carbon (i.e., atleast 95% carbon, more preferably, at least 98% carbon, most preferably,greater than 99.5% carbon) and has a density of typically less thanabout 1 g/cm³, preferably less than 0.5 g/cm³, more preferably less than0.3 g/cm³, which is suitable for thermal insulation. The insulationboard is sectioned or machined to an appropriate size for the desiredapplication. In the case of cylindrical castings, the casting can besectioned into several disks of a suitable thickness for a desiredapplication. Final machining of the disks can be used, for example, toform slots, grooves or other features in the disks. For board andsheets, final machining is used to provide the desired board dimensions.Optionally a sealant or coating is applied to the casting.

The cylindrical castings and board produced by this method are suited touse as rigid insulation materials, exhibiting good resistance to heatflow at high temperatures. For example, the castings are suited to useas insulation materials at temperatures of 1500–2000° C., or higher.Cylindrical castings having an average thermal conductivity of 0.13W/m-°K with a standard deviation of less than 0.05 W/m-°K, morepreferably, about 0.02 W/m-°K, or less, are readily formed by the abovedescribed centrifugal casting method. Board castings are readily formedwith a low density of 0.1 to 0.40 g/cm³, more preferably, from 0.15–0.25g/cm³, and a thermal conductivity of less than about 0.4 W/m-°K, morepreferably, about 0.16 to 0.3 W/m°K, making them desirable forlightweight thermal insulation products. Thermal conductivities aremeasured in air at 25° C., unless otherwise noted). High strengthlevels, greater than 180 psi (about 12.6 kg/cm²), are readily obtainedin such low density products. Where weight is not an important factor,higher density products are contemplated.

FIG. 4 shows an exemplary furnace, which includes an insulation shell 48assembled from an insulation board, formed according to the presentinvention. The boards are used to form side panels 50 and top and basepanels 52 for surrounding a furnace housing 54, such as an inductivelyheated graphite susceptor. A space 56 between the panels and the housingis preferably filled with a particulate or flexible insulation material,such as uncompressed particles 58 of expanded graphite. More than oneshell may be provided. For example, a second shell (not shown) maysurround and be spaced from the shell 48, the space being also packedwith insulation material similar to material 58. The panels are readilyremoved and/or replaced, due to their structural integrity, for example,when components of the furnace need to be repaired or replaced.

Without intending to limit the scope of the invention, the followingexamples demonstrate the properties of materials formed from isotropicpitch fibers and sugar as a binder.

EXAMPLES Example 1

Carbon fibers derived from either rayon, cotton, or isotropic pitch wereused as the reinforcement material. The isotropic pitch carbon fiberswere obtained from AnShan Chemical Co., China in three grade forms,P-400, P-600, and P-800, with the number designating the average lengthof the fibers in microns. The rayon fibers were produced by carbonizingraw rayon fibers to a temperature of about 800° C. and milling thecarbonized fibers to an average length of about 300 microns. Solutionsof cane sugar were prepared with concentrations ranging from 27–60%.

Slurries of fiber and sugar solution were filtered through a 12 cmdiameter ceramic filter funnel fitted with a filter cloth. The funnelwas attached to a tap water faucet providing a vacuum source. The binderwas pulled into a flask below the funnel. Extraction was ended when thebinder ceased to drip into the flask. The extracted disks were heated to250° C. to remove residual water and convert the sugar to an infusiblecured polymer. The dried disks were carbonized to about 1800° C. byheating in an inert atmosphere at a heating rate of 100° C./hour.

TABLE 1 shows properties of the carbonized products formed.

TABLE 1 SUGAR CONC. IN AQUEOUS SPECIFIC FLEXURAL THERMAL SOLUTION,DENSITY, RESISTANCE, STRENGTH, CONDUCTIVITY, FILLER WT % g/cm³ μΩm psiW/m-° K 50% UCR 60 0.23 689 234 0.28 rayon 1 + 50% UCR cotton 100% UCR60 0.18 954 180 0.20 Rayon 100% P-400 27 0.22 782 232 0.25 pitch 100%P-400 45 0.25 570 381 0.25 pitch 100% P-400 60 0.28 496 540 0.26 pitch100% P-600 27 0.22 975 244 0.25 pitch 100% P-600 45 0.25 757 260 0.25pitch 100% P-600 60 0.26 600 355 0.27 pitch 100% P-800 27 0.12 1495 810.16 pitch 100% P-800 45 0.15 1174 102 0.16 pitch 100% P-800 60 0.18 960188 0.16 pitch

As can be seen from TABLE 1, the pitch-based materials have low thermalconductivity, ranging from 0.16 to 0.27 W/m-° K, measured at 25° C.,which is excellent for thermal insulation purposes. The densities of0.12 to 0.28 g/cm are also well suited for providing lightweightinsulation materials. Very high strength levels were obtained for all ofthe pitch fibers, particularly at high binder concentrations. Ingeneral, the 45% and 65% binder compositions gave the best combinationof thermal conductivity, strength, and density.

Example 2

Insulation material prepared as large sheets suitable for furnace liningand the like were compared with a competitor's commercial insulationmaterial formed from rayon fibers and phenolic resin binder. TABLE 2shows that the isotropic pitch fiber/sugar-based composition had goodstrength and low ash content. The low density product had higherstrength (flexural and compressive) than a commercial product ofcomparable density.

TABLE 2 Insulation Insulation Material Material prepared by prepared bypresent present Competitor's process (“Low process (“High CommercialProduct Density”) Density”) Product Filler Isotropic pitch Isotropicpitch Rayon fibers fibers fibers Fiber weight %  85 Not known Averagefiber  1.2  1.2 (1.4) length, mm Binder Sugar Sugar Phenolic ResinDensity, g/cm³  0.19  0.27 0.16 (0.17 ± 0.02) Specific Resistance, WG719 WG 558 WG 1179 (1100) μΩm AG 5272 AG 1476 AG 4178 (4070) Flexuralstrength, WG 273 WG 383 WG 140 (149) psi AG 28 AG 88 AG 29 (22)Compressive WG 220 WG 418 WG 109 (160) strength, psi AG 185 AG 188 AG 91(110) Thermal WG 0.35 — WG 0.26 Conductivity in Air AG 0.15 AG 0.14 at25° C., W/m · K Thermal WG 0.77 0.65 (measured WG 0.55 Conductivity inAG 0.46 at 900° C.) AG 0.59 (0.47) Argon at 1000° C., W/m · K ThermalExpansion WG 3.08  3.0 WG (3.0 ± 0.3) 10⁻⁶/° C. at AG 3.1 AG — 1000° C.Thermal Expansion WG 5.8 — WG (5.6 ± 0.3) 10⁻⁶/° C. at AG 5.2 AG — 2000°C. Carbon Content, >99.9 >99.4 99.5 (>99.9) w/o Ash content, w/o  0.08 <0.6 0.45 (<0.07) ( ) indicates competitor's published data. All otherdata was established by comparative testing. WG = parallel to fiberorientation AG = perpendicular to fiber orientation

Example 3

Properties were measured during forming of isotropic pitch/sugar sheetproducts using a 60% sugar binder and compared with an equivalentproduct formed using a 27% sugar binder. TABLE 3 lists the resultsobtained.

TABLE 3 Sugar Green Binder Cure Calculated Density after density,Content, density, binder carbonization, g/cm³ w/o g/cm³ Content, w/og/cm³ 60% 0.37 51 0.31 23 0.27 sugar 27% 0.28 23 0.25 7 0.24 sugar

Example 4

The effects of fiber type on properties of insulation materials wereinvestigated in a pilot scale study. All of the materials were preparedusing a sugar binder. With the exception of the Rayon product, all wereprepared to a density of over 0.4 g/cm⁸. TABLE 4 compares the results.Of the higher density products, the isotropic pitch based product wasthe best in terms of thermal conductivity and flexural strength.

TABLE 4 FIBER DEN- FLEXURAL THERMAL LENGTH, SITY, STRENGTH,CONDUCTIVITY, FILLER μm g/cm³ psi W/m-° K Isotropic 370 0.45 705 0.45pitch (Kureha M- 104T) Mesophase 300 0.52 510 1.3 Pitch (Amoco VMX-11)Rayon Fiber 280 0.22 115 0.28 (UCAR) PAN fiber 3000 0.42 668 0.89(Zoltek Panex)

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. An insulation material formed by the method of: combiningcarbon-containing fibers having an average length of from about 400 toabout 800 micrometers, which include pitch-based carbon fibers, with abinder which includes a soluble sugar to form a mixture; forming themixture into a solid preform having a general shape of the thermalinsulation material; and heating the preform to a sufficient temperatureto carbonize the preform and form the thermal insulation material,wherein the thermal insulation material has a thermal conductivity ofless than about 0.4 W/m°K.
 2. A low density thermal insulation articleformed from a mixture of isotropic pitch carbon fibers having an averagelength of from about 400 to about 800 micrometers and a sugar binderwhich has been heated to a sufficient temperature to carbonize themixture, the article having a density of from about 0.1 to about 0.4g/cm3.